Polymerizable adhesive, coating and sealant compositions are well known. Typically, they comprise one or more reactive components, i.e., monomers and, optionally, comonomers, which react via condensation or addition polymerization reactions to form a cured or polymerized and/or, optionally, cross-linked, adhesive, coating or sealant. Phenols and formaldehyde and the reaction products thereof as well as amino compounds, especially ureas and melamines, and aldehydes, especially formaldehyde, and reaction products thereof are exemplary of the types of reactive components which react via a condensation type of polymerization. On the other hand, exemplary of the reactive components which polymerize, and/or cross-link via addition polymerization, there may be given vinyl compounds, epoxy compounds, unsaturated polyesters, poly- and mono-functional acrylate esters, styryloxy compounds, cyanoacrylate esters, mercapto and thiolene compounds, and urethanes as well as hybrids thereof e.g., polyacrylate-urethane, polyacrylate esters, thiolene-esters and the like.
Polymerizable compositions based on the foregoing reactive components may be either one-part or two-part polymerizable compositions and may or may not be carried in a solvent including, for example, water. Such polymerizable compositions may be self-reacting or may require the addition of a catalyst, initiator and/or accelerant to induce or enhance polymerization or curing of the same. In general, such polymerizable compositions may polymerize under aerobic or anaerobic conditions, as a result of their exposure to heat or to actinic radiation and the like. Furthermore, these polymerizable compositions may polymerize through two or more curing mechanisms or may comprise two or more polymerizable components each of which may cure independent of the other, i.e., by a different cure mechanism. For example, the polymerizable composition may contain a photopolymerizable component and an anaerobically polymerizable component. All of these facets of adhesive, coating and sealant technology, including specific formulations for the same, are well known and broadly disclosed in numerous publications and patents, including "Handbook of Adhesives", Erving Skeist, Ed. Reinhold Company 1977; "Advances in Adhesives" Brewis et al Ed. Warwick Publisher 1983; Encyclopedia of Polymer Science, H. F. Mark et al., Eds., 1987, and the numerous articles and patents referenced therein.
For the purpose of simplicity, hereinafter the term "polymerizable composition" shall refer to polymerizable and/or cross-linkable adhesive, coating and sealant compositions within the scope of the invention as described herein.
While many of the foregoing polymerizable compositions have excellent properties and characteristics, it is often desirable to further incorporate into said curable composition other additives which may enhance, modify or change the properties and/or characteristics of the cured or polymerized composition so as to be better suited for the many diverse applications for which they may be employed. Such additives include, but are not limited to, plasticizers, modifier resins and waxes, fillers, reinforcing agents, colorants, conductive agents, and the like, and are employed for such purposes as lower cost, higher strength, adhesion promotion, substrate wetability, greater heat resistance, flexibility, lower modulus, adhesion strength, low and elevated temperature performance, viscosity, less shrinkage, lower coefficient of thermal expansion, better electrical properties, and the like. Such additives, their purpose, and the amounts by which they are incorporated into the polymerizable compositions are well known.
Typically, the incorporation of additives into polymerizable compositions is as a dispersion thereof in the polymerizable composition. However, certain additives, especially liquid plasticizers, may be miscible with the polymerizable composition as well. Regardless, in order to obtain uniform performance from said additive modified polymerizable compositions, it is important that said additives be homogeneously dispersed therein. However, as a result of the different densities of the additives relative to the continuous phase or, with respect to miscible additives, within the continuous phase, there may be experienced a density drift or settling of the different constituents based on their density. This adverse effect is particularly troublesome for polymerizable compositions of low viscosity and long shelf or storage times. As a result of this density drift, as it is referred to herein, such polymerizable compositions manifest poorer and non-uniform properties unless the additive can be homogeneously dispersed once again. While vigorous shaking may recreate the homogeneous dispersion for polymerizable compositions of very low viscosity (highly liquid and flowable), such action is less successful for more highly viscous polymerizable compositions unless mechanically stirred for an extended time. Even so, these subsequent steps may not be feasible for a given polymerizable composition nor cost-effective. For example, polymerizable compositions containing an encapsulated activator, catalyst or accelerant may prematurely cure as a consequence of the rupture of the encapsulant particle upon vigorous shaking and/or mechanical mixing. Additionally, shearing stress caused by mechanical mixing may also adversely effect the reactive components and/or the additives e.g., rupturing of glass spheres, breaking of fibers and/or flake additives, etc.
Presently, attempts have been made to immobilize polymerizable compositions by incorporating therein a thixotropic agent. For example, colloidal silica, modified clays, and the like. Such thixotropic agents generally transform the polymerizable composition into a gel, thereby lessening the density drift, but not eliminating the same. Furthermore, unless high loading of said thixotropic agents are added, the gel formed may not be so stable as to greatly retard the progression of density drift, particularly upon long standing times. Also, it may not be desirable for the polymerizable composition to be in a gel form, as for example where high flowability is desired.
It is an object of the present invention to provide polymerizable compositions having constituents therein of different densities which do not drift or settle out even over long periods of time.
It is a preferred object of the present invention to provide polymerizable compositions having both a continuous polymerizable phase and a non-polymerizable discontinuous phase which do not suffer from density drift as a result of differing densities of the phases.
It is also an object of the present invention to provide density drift free, or essentially so, polymerizable compositions without permanently and significantly, if at all, altering the viscosity of the same.
Finally, it is also an object of the present invention to provide a method of preparing an immobilized polymerizable composition which is free or essentially free of density drift upon long term storage.